1. Field of the Disclosure
This invention is related to image sensors. In particular, embodiments of the present invention are related to pixels having floating diffusions.
2. Background
Image sensors have become ubiquitous. They are widely used in digital cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular, complementary metal-oxide-semiconductor (CMOS) image sensors, has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of these image sensors.
In a conventional CMOS active pixel cell, image charge is transferred from a photosensitive device (e.g., a photodiode) and is converted to a voltage signal inside the pixel cell on a floating diffusion node. The floating diffusion of each pixel cell is reset to a reset level before the image charge is transferred from the photosensitive device to the floating diffusion for each exposure. In general, a higher reset level in the floating diffusion helps to accommodate a larger full well capacity (FWC) in the floating diffusion, which helps to provide improved performance and image quality. In addition, it is appreciated that a higher floating diffusion reset level can also improve image lag performance of the pixel cell. A known solution to increase reset levels in floating diffusions is to add additional circuitry to pixel cells, which consequently results in additional costs and consumes valuable chip real estate.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.